The present invention relates to a system for adjusting the timing of the signal transmissions in time-division multiplexing signal transmission.
When a communication network is constructed by coupling a plurality of terminals (e.g., telephone sets) to a private branch exchanging equipment (PBX), it is generally difficult to directly couple the terminals to the PBX.
A communication network has been realized where a local station having concentration and distribution functions is installed in an individual building or on an individual floor of a building, and each local station is linked to a PBX (central station) through up and down links. Signals are sent from the local stations to the PBX on a time-division multiplexing basis. For this reason, a flexible network can be constructed.
In such a network, local stations are coupled at different positions to a signal transmission path (up and down links) coupled to the central station. For this reason, transmission path length between the central station and the local station is different for each local station. Different transmission path lengths mean different transmission delay times, i.e., different times required for signal transmission. When signals are transmitted from a plurality of local stations to the central station through a common transmission path, it is very important to prevent collision of transmission signals from the local stations. In order to prevent collisions, the signal transmission timing in each local station must be adjusted while taking different transmission delay times for each local station into consideration.
However, if extra time is allowed for signal transmission as a result of simple consideration of the transmission delay time of each local station, utilization efficiency of the transmission path would be considerably degraded.
An improved system for adjusting signal transmission timings in local stations is disclosed in the copending U.S. patent application Ser. No. 594,896 filed Mar. 29, 1984 now U.S. Pat. No. 4,594,705; entitled "BUS-CONFIGURED LOCAL AREA NETWORK WITH DATA EXCHANGE CAPABILITY"; and assigned to the same assignee as this application. With this improved system, a central station sends a test signal to local stations in succession, and the local stations sequentially send back the received test signal to the central station. The central station measures transmission delay time between itself and each local station, and provides transmission delay time data to each local station so as to adjust its signal transmission timing.
However, in this system, a heavy load is imposed on the central station since it must measure the transmission delay times of the local stations and control the signal transmission timings. With the delay time adjustment operation of the central station, the extension of the local stations is difficult, resulting in poor network flexibility. When a local station sends back the test signal, its transmission timing may include a quantum error caused by operations of the local station, thus impairing precision in delay time measurement. For this reason, a slight time margin must be taken into consideration in order to ensure prevention of collision of signals from local stations.